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explain why TOs were frequently observed in control animals that had not otherwise been
exposed to atrazine (Coady et al. 2004, 2005; Orton et al. 2006; Du Preez et al. 2009; Storrs-
Méndez and Semlitsch 2010). Notwithstanding the importance of phylogenetic variation
among test animals described above, the complete absence of hermaphroditic morpholo-
gies among control animals in several high-profile studies (Oka et al. 2008; Kloas et al.
2009a; Langlois et al. 2010a) perhaps suggests that under optimal conditions for growth
and development, hermaphroditic morphologies are less likely to persist in postmetamor-
phic subadults.
The studies by Kloas et al. (2009a) provide a case in point. These authors described the
results of two parallel studies in two separate laboratories that examined the effects of
atrazine (0.01 to 100 μg L
-1
) on the larval development of
X. laevis
. As indicated above, they
were unable to demonstrate any hermaphroditic morphologies in either control or exposed
animals. Their studies came about because of the methodological shortcomings indenti-
fied in prior studies (US EPA 2007). The criticisms placed particular importance on factors
relating to animal husbandry and biological loading in tests. Implicit within these criti-
cisms is an acknowledgment that other environmental co-stressors can affect the outcome
of toxicity bioassays (for a review of the effects of co-stressors, see Mann et al. 2009). It is
important to note that trade-offs between somatic and gonadal development have been
documented among
X. laevis
reared under conditions of environmental stress (McCoy et
al. 2007). In order to address these shortcomings, Kloas and colleagues adopted a stringent
feeding regime, and a flow-through system to ensure high water quality (Lutz et al. 2008).
In so doing, these authors provided a level of control over environmental parameters not
described previously in studies with amphibians, and perhaps provided conditions for
growth and development that were not available in studies where environmental con-
trols were lacking (e.g., the mesocosm studies of Jooste et al. 2005), or even available to
animals in the wild, which frequently breed in isolated ephemeral ponds where environ-
mental conditions can be extreme. In the wake of these landmark studies, the challenge for
amphibian ecotoxicologists was, and remains, to reintroduce environmental variability in
a manner that allows quantification of biotic and abiotic stressors and the effects they have
on the toxicity of environmental contaminants (Mann et al. 2009).
Despite the caution required when interpreting gonadal abnormalities in amphibians,
a few field studies continue to report disproportionately high rates of gonadal abnormali-
ties among frogs collected from agricultural zones (McCoy et al. 2008; McDaniel et al.
2008) and urban or industrial zones (Skelly et al. 2010). McDaniel et al. (2008) described a
high incidence (42%) of TOs among “adult [leopard] frogs and large juveniles (greater than
10 g)” collected from regions of intensive row agriculture in southern Ontario, Canada,
which was not observed in this same species (
R. pipiens
) collected from reference sites (7%).
Again, the emphasis was on leopard frogs, as the same phenomenon was not observed
among green frogs (
Rana clamitans
) that displayed TOs among 10% and 11% of male frogs
sampled from nonagricultural and agricultural sites, respectively.
McCoy et al. (2008) described an increased prevalence of gonadal abnormalities in
giant toads (
Bufo marinus
; known in Australia as cane toads) along a gradient character-
ized by increasing agricultural intensity. All abnormalities described for these toads were
based on gross anatomical features that were not confirmed by histology. Abnormalities
included mixed gonadal morphologies such as presence of both male and female repro-
ductive structures (i.e., testes as well as vitellogenic or nonvitellogenic ovaries and/or ovi-
duct) or abnormal development of the Bidder's organ (i.e., multiple or vitellogenic Bidder's
organs without ovaries or oviducts). Histological examination of these tissues would likely
have added an extra layer of valuable data, and no doubt confirmed the coexistence of
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